Display Item Actions on show views

[Flo0807]

Item Actions should be displayed on show pages by default.

Todos

• Display Item Actions on show view by default
• Handle Item Actions clicked on show view (you might need to handle some edge cases)
• Allow :show key to be added to only configuration to restrict placement
• Update docs
  • Document the new :show key that can be used in only option to configure placement (see https://hexdocs.pm/backpex/0.10.0/item-actions.html#placement-of-item-actions)

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I saw this entry PR #591 from six month ago which pretty much the feature I'm after.

I would like to show specific item actions on the show page instead of the index. Is there a way to do this at the moment?

Originally posted by @armanm in #771

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Add a "delete" button in view and edit screen (on the side of "edit" button on right top of view screen).

Originally posted by @thothix in #898